Login Error: Can't acquire directory lock

I am currently unable to login to our DOORS server. When I run DOORS, the splash screen appears and everything appears to work fine, but instead of the login dialog box appearing, an error box occurs that says:

Unable to read the user directory: Unable to acquire directory lock. Please try again later or contact your database manager.

Has anyone encountered this before? What could be the cause?
Peter.Dawson - Mon Dec 21 21:11:59 EST 2009